Chemically Passivated Zinc Oxide Photoelectrode for Photoelectrochemical Water Splitting

ABSTRACT

A chemically passivated photoelectrode, having a conductive substrate, a layer of conductive oxide, preferably zinc oxide (ZnO), over the conductive substrate, and an ultrathin layer of a chemically inert semiconductor material coating the conductive oxide layer, is disclosed. The ultrathin layer of chemically inert semiconductor material, which may be less than 5 nm thick, increases the efficiency of water splitting through passivation of surface charge traps and chemical stability in harsh environments, as opposed to being photoactive. A method of manufacture and a solar cell having the photoelectrode are also disclosed.

CROSS-REFERENCE TO A RELATED APPLICATION

This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61/898,893 filed on Nov. 1, 2013, the content of which is incorporated herein in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

The present invention was made with government support under contract number DE-AC02-98CH10886 awarded by the U.S. Department of Energy. The United States government has certain rights in the invention.

BACKGROUND

I. Field of the Invention

This disclosure relates to the field of photoelectrochemical water splitting. In particular, the disclosure relates to a chemically passivated photoelectrode for use in photoelectrochemical water splitting, and a method of chemically passivating photoelectrodes for use in water splitting.

II. Background

Photoelectrochemical water splitting, also known as artificial photosynthesis, has long been suggested as a promising way to capture and store solar energy. The heart of a typical water splitting cell is a semiconductor photoelectrode, in which electron-hole pairs are generated upon light absorption. Following charge carrier separation, the electrons reduce water to hydrogen while the holes oxidize water to oxygen.

However, the energy conversion efficiency for photoelectrochemical water splitting remains low due to multiple material limitations. For example, conventional photoelectrodes fabricated with zinc oxide (ZnO) nanostructures (Greene, L. E. et al. Angew. Chem., Int. Ed. 42, 3031-3034 (2008); Greene, L. E. et al. Nano Lett. 5, 1231-1236 (2005); Shim, M et al,. J. Am. Chem. Soc. 123, 11651-11654 (2001); each of which is incorporated by reference in its entirety) generally contain large defect populations such as oxygen vacancies that act as deep traps. The deep traps directly block the interfacial electrochemical reaction, significantly reducing the energy conversion efficiency. Moreover, ZnO is only marginally stable in aqueous solution and dissolves in either acidic or alkaline environment (Yang, X. Y et al. Nano Lett. 9, 2331-2336 (2009); Ahn, K. S et al., Appl. Phys. Lett. 91, 231909 (2007); Ahn, K. S. et al., Appl. Phys. Lett. 93, 163117 (2008), each of which is incorporated by reference in its entirety). As a result, photoelectrochemical water splitting with ZnO is typically performed in neutral pH electrolyte solutions, where the absence of H⁺ and OH⁻ increases the electrolyte resistivity and significantly limits mass transport near the semiconductor/electrolyte interface.

While previous strategies of dealing with these shortcomings have included either treating crystalline or polycrystalline semiconducting oxides with methods such as electrochemical compensation, and coating highly conductive materials with TiO₂ shells (see, for example, U.S. Pat. No. 4,511,638, U.S. Pat. No. 8,216,436 and U.S. Patent Publication No. 2010-0043877 A1, each of which is incorporated by reference in its entirety), the use of TiO₂ as a photoactive component results in much lower conversion efficiency as TiO₂ has fundamental deficiencies such as a wide band gap, low carrier densities, and poor conductivity.

There remains a need for a semiconductor electrode that is chemically robust and able to withstand the harsh condition for water oxidation/reduction.

SUMMARY

In view of the aforementioned challenges, chemically passivated photoelectrodes are provided. The photoelectrodes have an electronic band gap small enough to maximize solar light absorption while still providing sufficient energy to drive a water splitting reaction, while possessing suitable electrical conductivity to facilitate charge carrier separation with minimal ohmic loss. The disclosed chemically passivated photoelectrode has a conductive substrate, a layer of conductive oxide, such as zinc oxide (ZnO), over the conductive substrate, and an ultrathin layer of a chemically inert semiconductor material coating the conductive oxide layer. The ultrathin layer of chemically inert semiconductor material, which is less than 5 nm thick, increases the efficiency of water splitting through passivation of surface charge traps and chemical stability in harsh environments, as opposed to being photoactive. In another embodiment, the conductive oxide is cuprous oxide.

In certain embodiments, the chemically inert semiconductor material of said ultrathin layer is selected from the group consisting of titanium dioxide, hafnium dioxide, and zirconium dioxide.

In certain embodiments, deep trap states in the conductive oxide, such as ZnO, layers are removed through treatment with reactive plasma, thermal annealing, or a combination thereof, prior to the deposition of the chemically inert semiconductor material. In an embodiment, the reactive plasma is oxygen plasma. In yet another embodiment, the conductive oxide, such as ZnO, layer is annealed at a temperature that ranges between about 250° C. and about 500° C. in O₂, such as at about 500° C.

In yet another embodiment, the conductive oxide, preferably ZnO, layer may comprise nanostructures such as nanowire arrays. The nanostructures may be doped with foreign elements such as nitrogen, gallium, and indium, amongst others, in order to lower the band gap of the conductive oxide, thereby increasing the conversion efficiency of photoelectrochemical water splitting.

A solar cell, which includes the chemically passivated photoelectrode, is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a photoelectrode according to an embodiment of the invention.

FIG. 1B is a schematic diagram o of a photoelectrode according to an embodiment of the invention. The inset illustrates an ultrathin layer of TiO₂ coating the ZnO nanostructures.

FIG. 1C is a schematic diagram of a solar cell comprising a photoelectrode according to an embodiment of the invention.

FIG. 2A is an SEM image of a ZnO nanowire array grown on an indium tin oxide (ITO) substrate.

FIG. 2B is a TEM image of a ZnO nanowire, with the inset showing a selective area electron diffraction (SAED) pattern of the nanowire.

FIG. 2C is a photoluminescence spectra of the ZnO nanowire array (sample I) compared to the ZnO core/TiO₂ shell nanowire array (sample II). The inset is a schematic energy diagram of the ZnO nanowire, showing the location of the deep hole traps. CB, VB, and EF are the conduction band edge, valence band edge, and ZnO Fermi level, respectively.

FIG. 2D is an energy-dispersive X-ray spectroscopy (EDX) elemental line scan of a ZnO core/TiO₂ shell nanowire (shown in the inset).

FIG. 3 schematically illustrates various treatments of as-grown ZnO nanowires (sample I) with: (II-control no pretreatment; III-oxygen plasma; and, IV-thermal annealing and oxygen plasma) followed by TiO₂ deposition to produce samples II-IV.

FIG. 4A is a plot showing current density as a function of potential in the dark and in simulated AM 1.5 illumination for the ZnO nanowire array during the initial test and after 1 hour of illumination of the untreated, as-grown Zno nanowire arrays. The thermodynamic potential for oxygen evolution (E°(O₂|H₂O)) is indicated by a dashed vertical line, and the top axis is presented as the potential vs. RHE (reversible hydrogen electrode). The electrolyte is a 0.1 M KOH aqueous solution with pH=13.

FIG. 4B is a plot showing current density as a function of potential in the dark and in simulated AM 1.5 illumination for the untreated ZnO core/TiO₂ shell nanowire array during the initial test and after 1 hour of illumination. The thermodynamic potential for oxygen evolution (E°(O₂|H₂O)) is indicated by a dashed vertical line, and the top axis is presented as the potential vs. RHE (reversible hydrogen electrode). The electrolyte is a 0.1 M KOH aqueous solution with pH=13.

FIG. 5A is a plot showing the photoluminescence spectra of the ZnO core/TiO₂ shell nanowire array that has the ZnO core plasma cleaned but unannealed (sample III) and the one that has the ZnO core thermally annealed and plasma cleaned (sample IV), illustrating the significantly more intense band edge photoluminescence emission at 3.3 eV in the sample that is treated with oxygen plasma (sample III).

FIG. 5B is a plot showing current density as a function of potential in simulated AM 1.5 illumination for sample III and sample IV. The dark j-E curves for each sample are shown as dotted lines. The thermodynamic potential for oxygen evolution (E°(O₂|H₂O)) is indicated by a dashed vertical line and the top axis is presented as the potential vs. RHE. The electrolyte is a 0.1 M KOH aqueous solution with pH=13. The results illustrate a 20% increase of photocurrent at zero overpotential, and a 30% increase in water splitting energy conversion compared to sample II (FIG. 4B).

DETAILED DESCRIPTION

The specification discloses, in one embodiment, a chemically passivated photoelectrode that is substantially resistant to photocorrosion and/or deterioration in harsh environments. Methods for fabricating the chemically passivated photoelectrode are disclosed. A solar cell, which includes the chemically passivated photoelectrode, is also disclosed.

Specifically, referring to FIG. 1A, a chemically passivated photoelectrode 100, which includes a conductive substrate 101, a layer of ZnO 102, over the conductive substrate 101, and an ultrathin layer of chemically inert semiconductor material 103, which is less than 5 nm thick, coating the layer of ZnO 102. The layer of chemically inert semiconductor material may be TiO₂, but may include other semiconductor oxides, such as Hafnium Dioxide (HfO₂) and Zirconium Dioxide (ZrO₂).

The thickness of the inert semiconductor shell may affect the performance and the stability of the photoelectrode. It has been found, that when the semiconductor shell is thicker than 5 nm, the TiO₂ coating behaves as a photoactive component, resulting in much lower conversion efficiency than when the semiconductor shell is thinner than 5 nm. TiO₂ has fundamental deficiencies such as a wide band gap, low carrier densities, and poor conductivity. A coating of TiO₂ or other inert semiconductor materials having a thickness of less than 5 nm results in superior performance as ZnO, which is naturally a better conductor of both electrons and holes, acts as the photoactive component. In an alternate embodiment, the photoactive component that is coated with an ultrathin layer of chemically inert semiconductor may include other materials known in the art, such as cuprous oxide (Cu₂O). The ultrathin coating serves to passivate the ZnO layer and provide protection against photocorrosion when placed in electrolyte 104.

In the disclosed photoelectrode, the conductive substrate 101 can be any substrate suitable for supporting a conductive oxide film. The conductive substrate 101 may be rigid or flexible. Substrates include, but are not limited to, glass, gold, aluminum, steel, silicon wafers, SiO₂, and polyimide.

A high surface to volume ratio provides advantages in photoelectrochemical water splitting. For example, the high surface to volume ration results in increased contact with the electrolyte. As shown in FIG. 1B, the ZnO layer 102 may comprise one or more vertically oriented nanostructures 105. The nanostructures 105 may be synthesized by any method known in the art (see, for example, U.S. Pat. No. 7,545,051, which is hereby incorporated by reference). The one or more nanostructures 105 may be selected from nanowires, nanorods, nanotips, and combinations thereof. The one or more nanostructures may vary in diameter and length. The diameter may be between about 10 nm 100 nm, such as about 50 nm. The length may be between about 500 nm to 1000 nm.

One or more dye and/or polymer molecules may be bonded to the nanostructures 105 prior to the deposition of the chemically inert semiconductor layer 103 to enhance the performance of the photoelectrode 100 in electrochemical water splitting and other electrolytic processes. Suitable dyes and polymers may include those that enhance light absorption by the photoelectrode 100. Such dyes include, but are not limited to, Ru-based, Fe-based, and phthalocyanine-based dyes. Polymer molecules may include, but are not limited to, poly(alkyl)thiophene, polyparaphenylene vinylene, polyvinyldifluoride, and copolymers thereof.

The ZnO layer 102 may be doped with foreign elements in order to lower its band gap in order to increase efficiency of photoelectrochemical water splitting. Dopants may include, but are not limited to, Group-III elements such as boron, nitrogen, gallium, and indium.

In one embodiment of the disclosed invention, the ZnO layer 102 is treated with oxygen plasma prior to the deposition of the chemically inert semiconductor material. In another embodiment, the ZnO layer 102 is thermally annealed in O₂ at a temperature of about 250° C. to about 500° C. prior to the deposition of the chemically inert semiconductor material. In an embodiment, the ZnO layer 102 is both treated with oxygen plasma and thermally annealed in O₂ at a temperature of about 250° C. to about 500° C., such as at 500° C., prior to the deposition of the chemically inert semiconductor material. Treatment of the ZnO layer 102 prior to the deposition of the chemically inert semiconductor layer 103 results in the passivation of surface charge traps and the removal of deep trap states, further improving water splitting and efficiency and other electrochemical processes. Other electrochemical processes may include photocatalytic activity, which can be used, for example, to decompose organic pollutants in water.

The thin layer of chemically inert semiconductor material deposited over the ZnO layer 102 can be applied by various chemical or physical means known in the art. Methods of deposition may include, but are not limited to, atomic layer deposition (ALD) and chemical vapor deposition (CVD). In an embodiment, the chemically inert semiconductor material is TiO₂.

Also disclosed is a solar cell 110 (FIG. 1C), which includes the disclosed chemically passivated photoelectrode 100. The solar cell 110 includes a photoelectrode 100 with a conductive substrate 101 and a nanostructured semiconductor oxide layer 102, liquid redox electrolyte 104, and a counter-electrode 106. The disclosed photoelectrode provides a chemically passivated photoelectrode 100 that is resistant to photocorrosion in an aqueous solution under ultraviolet illumination. The disclosed photoelectrode 100 is also resistant to corrosion in alkaline solutions, as alkaline solutions are generally preferred for water splitting photoanodes as the environment provides adequate OH⁻ ions adsorbed over the electrode surface to receive the photogenerated holes.

Solar cells include, but are not limited to, dye-sensitized solar cells, polymer solar cells, silicon solar cells, copper indium gallium selenide (CIGS) solar cells, bulk-heterojunction solar cells, and hybrid inorganic-organic solar cells.

The disclosed photoelectrode may be fabricated using any technique that is well known in the art. First, a nanowire array is grown on a conductive substrate. The growth of a nanowire array comprises a deposition step and a growth step. The deposition step comprises depositing nanocrystals on the substrate using any suitable deposition method. Suitable methods may include, but are not limited to self assembly processes, spin coating, spraying, roller coating, dip coating. The growth step, which occurs after the deposition step, comprises contacting the substrate, now seeded with nanocrystals, with a growth medium. The growth medium may comprise any suitable material that induces growth of nanowires, which may include, but are not limited to amines, phosphonic acids, and carboxylic acids.

Following the growth of the nanowire array over the conductive substrate, a chemically inert semiconductor material coating is deposited over the nanowire array. Chemical and physical means of deposition are well known in the art. Suitable deposition methods may include, but are not limited to, chemical solution deposition, spin coating, chemical vapor deposition, atomic layer deposition, molecular beam epitaxy, and sputtering.

The completed photoelectrode may be used in solar cells. When used in solar cells, the disclosed photoelectrode is connected to external circuits and immersed into an aqueous electrolyte solution. The electrolyte solution may be an alkaline solution as the environment provides adequate OH⁻ ions adsorbed over the electrode surface to receive the photogenerated holes.

The following non-limiting examples set forth herein below illustrate certain aspects of the invention.

EXAMPLES Example 1 Synthesis of ZnO Core/TiO₂ Shell Nanowire Array

A ZnO nanowire array is grown over an indium tin oxide (ITO) glass substrate using a seed-mediated hydrothermal method described in Greene, et al. (Angew. Chem., Int. Ed. 2003, 42, 3031-3034; Nano Lett. 2005, 5, 1231-1236). To deposit the ZnO seed, an ethanol solution of zinc acetate is drop-cast over the growth substrate, then baked at 325° C. for 20 minutes in air. The seeded substrate is then placed in an aqueous solution of zinc nitrate and hexamethylenetetramine that is heated to 90° C. After 120 minutes, the nanowire array is thoroughly washed with deionized water and dried in a stream of nitrogen (N₂) to yield Sample I as illustrated in FIG. 3. A TiO₂ shell is coated over the ZnO nanowire array in a Cambridge Nanotech Savannah S 100 atomic layer deposition (ALD) system, using titanium isopropoxide and water as precursors. The deposition is carried out at 250° C. for 100 cycles (nominal deposition rate 0.01 nm/cycle), under 300 mTorr of nitrogen carrier gas flowing at 20 sccm. Prior to the TiO₂ coating, various processing techniques, including thermal annealing and oxygen plasma cleaning, are applied to the as-grown ZnO nanowire arrays. The morphology and compositions of the nanowires are characterized using a Hitachi S-4800 scanning electron microscope and a JEOL JEM2100F transmission electron microscope.

The as-grown ZnO nanowires (FIG. 3, Sample I) have an average diameter of 40 nm and average length of 0.55 μm (FIG. 2A), with a packing density of approximately 2.2×10¹⁰ cm⁻² and a single crystalline wurtzite structure with c-axis growth direction (FIG. 2B). Because of their high aspect ratio and density, the nanowire arrays provide approximately 15 times, more surface area than the planar substrate.

Example 2 Photoluminescence Measurements

Photoluminescence (PL) spectra of the nanowire arrays are measured in an ISS PC1/K2 spectrofluorometer that uses a xenon lamp for optical excitation and a photon multiplier tube for PL detection. The excitation wavelength is set to 280 nm by a monochromator. The PL emission is collected by a second monochromator at the normal direction to the excitation beam. A 320 nm long-pass filter is inserted between the sample and the second monochromator to remove the scattered excitation light.

The steady-state photoluminescence (PL) spectra of as-grown nanowire arrays show weak band gap emission near 3.3 eV (376 nm), accompanied by a much stronger deep-level emission at approximately 2.2 eV (560 nm) (FIG. 2D) that is generally associated with deep hole traps due to oxygen vacancies with energies approximately 1 eV above the valence band edge. The deep level emission is significantly suppressed after the ZnO nanowire arrays are coated with a nominal 1 nm thick TiO₂ shell using atomic layer deposition (ALD) at 250° C. The core/shell nanostructure shows stronger band gap emission and reduces deep-level emission by nearly 90%, based on the change in integrated PL intensity, compared to an uncoated ZnO nanowire array (FIG. 1C), consistent with passivation of bare ZnO surface states.

Example 3 Electrochemistry Measurements

The measurements are performed on a measurement station equipped with a potentiostat (VERASTAT, Princeton Applied Research, Oakridge, Tenn.), a custom-built three-electrode quartz-windowed photoelectrochemical cell, and a 150 W solar simulator with AM 1.5 G filter (Newport Corporation, Stratford, Conn.). In the photoelectrochemical cell, 0.1 M KOH solution was used as an electrolyte, the nanowire array substrate as a working electrode (active/uniform illumination area of 1.0 cm²), an Ag/AgCl/3 M KCl reference electrode (0.210 V_(NHE)), and a platinum wire counter electrode. The incident light power is calibrated by a calibrated quartz-windowed Si solar cell (Newport) and a spectrometer calibrated for an absolute irradiance measurement (Ocean Optics). For the incident photon-to-current efficiency (IPCE) measurement, we use a 300 W xenon arc lamp and a grating monochromator equipped with band-pass filters for removing higher order diffractions. The light power for each wavelength is measured by an optical power meter (Newport 1918-C) and a UV-enhanced Si photodiode sensor.

Example 4 Photoelectrochemistry Measurements

Linear current density-potential (j-E) sweeps (FIG. 4A) were performed for the as-grown ZnO nanowire array, in the dark and under simulated 1 sun (AM 1.5 G) illumination conditions. Negligible anodic dark current was measured until applying about 0.6 V overpotential vs E°(O₂|H₂O), the thermodynamic potential for oxygen evolving (dashed vertical line, FIG. 4A). E°(O₂|H₂O) is calculated from the Nernst relation, E°(O₂|H₂O)=(1.23−0.05917 pH) V_(NHE)=0.46 V_(NHE), for pH=13, which is 0.25 V vs the AgCl/Ag reference electrode (0.210 V_(NHE)). The illuminated cell produced significant anodic photocurrent with an onset potential at −0.55 V_(AgCl/Ag), which roughly represents the flat band potential (E_(fb)) of the system.

The anodic photocurrent onset at 0.8 V negative of E°(O₂|H₂O) indicates the photoelectrochemical system is capable of harvesting solar energy through water splitting. Higher applied voltage results in further band bending at the ZnO/electrolyte interface, improved charge carrier separation efficiency, and higher photocurrent. At zero overpotential vs E°(O₂|H₂O), the photocurrent output of the as-grown ZnO nanowire array reaches 0.40 mA/cm², which is among the highest reported photocurrents for a ZnO-based photoelectrochemical water splitting cell (FIG. 4A). The alkaline electrolyte (0.1 M KOH) supplies a large OH⁻ concentration supporting the anodic reaction, 4OH⁻+4h⁺→O₂+2H₂O, resulting in higher output. Similarly ZnO nanowire arrays show about 20 times less photocurrent (0.02 mA/cm²) in a pH neutral electrolyte, under otherwise identical conditions. The photoelectrochemical water splitting energy conversion efficiency (η) of the as-grown ZnO nanowire array reaches 0.09% at −0.1 V_(AgCl/Ag), when calculated by the following relation:

$\eta = {j_{p}\frac{{E\; {^\circ}\mspace{11mu} \left( O_{2} \middle| {H_{2}O} \right)} - E}{I_{0}} \times 100\%}$

where j_(P) is the photocurrent density, (E°(O₂|H₂O)−E) is the electrode underpotential vs E°(O₂|H₂O), and I₀=100 mW/cm² is the 1 sun (AM 1.5 G) incident light power. The photocurrent output decreases by about 10% after 1h of the photoelectrochemical reaction (FIG. 4A), with a corresponding decrease in η to 0.08%, due to the slow dissolution of ZnO in the strong alkaline electrolyte.

Example 5 TiO₂ Coating Increased Stability and Performance

The photoelectrochemical water splitting activity and chemical stability of the ZnO nanowire anode are significantly improved after TiO₂ coating and additional processing, which are summarized in FIG. 3. The ZnO core/TiO₂ shell nanowire array (Sample II as shown in FIG. 3) has qualitatively similar photoelectrochemical j-E characteristic (“initial” curve in FIG. 4B), but with about 25% larger photocurrent (0.5 mA/cm²) at zero overpotential, compared to as-grown ZnO wires. The maximum energy conversion efficiency improves to 0.10% (at −0.1 V_(AgCl/Ag)). Importantly, during the course of 1 h, there is negligible change in the j-E characteristic, implying that the chemically inert TiO₂ shell formed by ALD improves ZnO nanowire stability. Measurement over longer duration (≧3 h) does not lead to decay in performance or visible damage to the photoanode. Because the TiO₂ shell is very thin (about 1 nm) compared to the ZnO nanowire diameter (about 40 nm), the improved performance is not due to additional light absorption in the shell, which was reported for the case of a much thicker TiO₂ coating. Instead, it is consistent with the partial removal of surface trap states as observed in photoluminescense. The ultrathin shell also minimizes the path for the minority carriers (photoholes) to travel from the bulk of ZnO to the photoanode surface.

Example 6 Removal of Deep Trap Surface States

Further removal of deep trap ZnO surface states enabled substantial enhancement in the photoelectrochemical activity of ZnO core/TiO₂ shell nanowire arrays. Treatment of as-grown ZnO nanowire arrays with an oxygen plasma prior to coating with a thin TiO₂ shell (Sample III as shown in FIG. 3) lead to significantly more intense band edge photoluminescence emission at 3.3 eV (376 nm), which originates from the removal of nonradiative charge traps, i.e., dark states (FIG. 5A). Compared to Sample II (FIG. 3), photoelectrochemical cells made from these structures produce a 20% higher photocurrent of 0.61 mA/cm² at zero overpotential (FIG. 5B), as well as a 30% improvement in η_(max)=0.13% (at −0.14 V_(AgCl/Ag)). When compared to the as-grown ZnO nanowires (Sample I as shown in FIG. 3), the photocurrent improved by 52% and the energy conversion efficiency improved by 44%. The increase in energy conversion efficiency results from both increased photocurrent (at zero overpotential) and a higher fill factor (FF) of the j-E curve. The higher FF reflects reduced charge carrier recombination within the ZnO.

Annealing as-grown ZnO nanowire arrays in O₂ at 500° C. before performing the O₂ plasma treatment and growing the TiO₂ shell (Sample IV as shown in FIG. 3) renders the photoluminescence spectrum nearly free of deep level emission (FIG. 5A) and further improves its photoelectrochemical water splitting activity. Such photoanodes produce photocurrent of 0.7 mA/cm² at zero overpotential (FIG. 5B), with η_(max) of about 0.17% (at −0.18 V_(AgCl/Ag)),representing 75% and 89% improvements over the photocurrent and efficiency, respectively, of the as-grown ZnO nanowires (Sample I, FIG. 3). The values of zero-overpotential photocurrent and maximum energy conversion efficiency are 75% and 55% higher than any previously reported values for photoelectrochemical water splitting cells based on a ZnO photoanode. The stabilities of Samples III and IV are similar to Sample II, again confirming the effectiveness of the TiO₂ shell in protecting the nanowire photoanodes.

The foregoing example and description of the preferred embodiments should be taken as illustrating, rather than as limiting the present invention as defined by the claims. As will be readily appreciated, numerous variations and combinations of the features set forth above can be utilized without departing from the present invention as set forth in the claims. Such variations are not regarded as a departure from the spirit and script of the invention, and all such variations are intended to be included within the scope of the following claims. 

1. A photoelectrode comprising a conductive substrate, a layer of conductive oxide over said conductive substrate, and a layer of chemically inert semiconductor material coating said conductive oxide layer, wherein said layer of chemically inert semiconductor material coating has a thickness of less than 5 nm.
 2. The photoelectrode of claim 1, wherein the conductive oxide layer is zinc oxide or cuprous oxide.
 3. The photoelectrode of claim 1, wherein the conductive oxide layer is zinc oxide.
 4. The photoelectrode of claim 2, wherein the conductive oxide layer comprises conductive oxide nanostructures.
 5. The photoelectrode of claim 4, wherein the nanostructures are selected from the group consisting of nanowires, nanorods, and nanotips.
 6. The photoelectrode of claim 4, wherein the conductive oxide nanostructures comprise oxygen plasma cleaned conductive oxide nanostructures.
 7. The photoelectrode of claim 6, wherein the cleaned conductive oxide nanostructures are thermally annealed at a temperature between about 250° C. and about 500° C. in O₂.
 8. The photoelectrode of claim 7, wherein the cleaned conductive oxide nanostructures are thermally annealed at a temperature of about 500° C. in O₂.
 9. The photoelectrode of claim 1, wherein the chemically inert semiconductor material is selected from the group consisting of TiO₂, HfO₂, and ZrO₂.
 10. The photoelectrode of claim 1, wherein the chemically inert semiconductor material is TiO₂.
 11. The photoelectrode of claim 1, wherein the thickness of the chemically inert semiconductor material layer is about 0.9 nm to about 5 nm.
 12. The photoelectrode of claim 11, wherein the thickness of the chemically inert semiconductor material layer is about 1 nm.
 13. The photoelectrode of claim 1, wherein the conductive oxide layer is doped with one or more foreign elements.
 14. The photoelectrode of claim 13, wherein the foreign elements are selected from the group consisting of nitrogen, gallium, and indium.
 15. A solar cell comprising a photoelectrode of claim
 1. 16. A method of fabricating a photoelectrode, comprising depositing a layer of a conductive oxide material over a conductive substrate; growing nanostructures of said conductive oxide material over said layer of said conductive oxide material; and depositing an ultrathin layer of a chemically inert semiconductor material over said layer of conductive oxide material.
 17. The method of claim 16, wherein the layer of conductive oxide material is thermally annealed at a temperature between about 250° C. and about 500° C. in O₂ prior to the deposition of the layer of chemically inert semiconductor material.
 18. The method of claim 17, wherein the layer of conductive oxide material is thermally annealed at a temperature of about 500° C.
 19. The method of claim 17, wherein the layer of conductive oxide material is treated with oxygen plasma prior to thermal annealing.
 20. The method of claim 19, further comprising the step of thermally annealing the layer of conductive oxide material at a temperature between about 250° C. and about 500° C. in O₂ prior to the deposition of the layer of chemically inert semiconductor material. 